A liquid crystal display displays images by changing arrangement of liquid crystal molecules by applying and withdrawing an electric field and thereby controlling transmission/blocking of light. The liquid crystal display, in principle, does not require a large amount of power for driving, and it is a display device with a low power consumption, in which power consumption is maintained small. Thus, liquid crystal displays are widely used as output displays of portable terminals, particularly those mainly powered by batteries, such as cellular phones and PDAs (personal digital assistants).
In a liquid crystal display for this type of application, in order to allow use of a battery for a long period by a single charging, attempts have been made to reduce power consumption by lowering the driving voltage or by lowering the driving frequency. Furthermore, as a pixel structure that allows further reduction in power consumption, a liquid crystal display in which a memory is provided for each pixel is known (for example, refer to Japanese Unexamined Patent Application Publication No. 9-212140).
In such a liquid crystal display of a pixel structure in which a memory is provided for each pixel, with regard to a still picture, once image data is written to a memory unit of a pixel, it suffices to repeatedly drive the pixel for display using the image data stored in the memory of the pixel. Thus, signal lines need not be charged and discharged on each occasion, and in principle, the only electric power required is that for inverting polarity. This allows further reduction in power consumption.
In the liquid crystal display having the construction described above, the arrangement has hitherto been such that the same path is used for writing image data from a signal line to a memory unit of a pixel and for reading image data out of the memory into a liquid crystal cell unit at the pixel. Thus, when image data is written to the memory, since the liquid crystal cell unit is connected to a write line and the pixel capacitance is charged, the potential of the liquid crystal cell unit (hereinafter referred to as pixel potential) becomes unstable, affecting the write operation. Consequently, depending on characteristics of transistors forming the pixel circuit, data held in the memory might be modified by the pixel potential, causing considerable variation in picture quality due to the variation in the transistor characteristics.
The present invention has been made in view of the above problem, and an object thereof is to provide a liquid crystal display in which the effect of a pixel voltage during writing of data to a memory is removed, serving to provide a large margin against variation in characteristics of transistors forming a pixel circuit, and to provide a portable terminal having the liquid crystal display.